Experience with Cast Epoxy Resin Instrument Transformer Catastrophic Failure (72kV) 1

[j_p_c]

Hi All,

Thanks in advance for your thoughts. I post once every few years or so, but read often. Application is in North America.

We had a brand new 72 kV dry-type cast epoxy resin voltage transformer fail catastrophically recently. No personnel were harmed, but easily could have been. Collateral damage to surrounding yard equipment was significant, leading to a long down time period for the client.

In discussion w. other industry colleagues, I've now been hearing of other epoxy type VTs that have failed in a dramatic fashion --> heavy, far flung projectiles.

We were unable to determine the root cause of the failure, so I wanted to reach out to the ET community to see if any one else has seen this, or has done or found research on the matter.

The voltage transformers were installed as a replacement to oil filled CVTs (functioning perfectly well) to meet a utility specification for two secondary voltage winding ratings that would meet their revenue meter input voltage required rating (115V instead of 69V), it was a straightforward replacement with the utility specifying the unit as one of their standards.

After initial energization (~2h) one of the phases began to boil off its resin and smoke, and within minutes exploded catastrophically.

Before energization, all three units tested well under NETA site acceptance criteria (insulation resistance, ratio, winding resistance, secondary burden) and the accompanying comprehensive factory results were all excellent. Power factor testing did not give intelligible results, this was expected as per the manufacturer for this dry type insulation system. We did not do an induced (from the secondary) voltage test of the primary, as it is an optional test. An applied (withstand) voltage test of the primary was not possible as the neutral was insulated to ground.

A potential influencing factor is that it was discovered during startup that the instrument secondary neutral ground wire had been lifted by the contractor and not fully replaced, they had signed off on tighten and tug tests and then went in to address a deficiency and tape the ground connection green, and it seems had disconnected the wire to tape it and replace it, rather than just taping in place. The unlanded ground was found because the line relays were indicating mild imbalance in the measured secondary voltages (off by ~15%), once the circuit was isolated, and the wire landed voltages sprang back to expected nominal. The mild imbalance was measured both at the relay HMI, and via handheld meter, phase to cabinet ground.

Notes:
- the H2 bonding jumper to ground was properly
- VTs are wye grounded-wye grounded, and on the utility line side of customer delta high side transformer winding. Utility is solidly grounded from their end.
- identical replacement VT units are now installed and the customer is back up and running again
- the other two units that were installed next to the failed one were fully factory re-tested and passed well.
- no fusing on the VT primaries
- line relays detected a steady reduction in failed VT phase voltage (indicating a steady increase in ratio) over the course of 5-10 minutes, before tripping the load end at 0.8 pu (not the utility end unfortunately), total failure was 5-10 mins after that.
- the after effect look of the VT is similar to that shown in this paper:

http://ieeexplore.ieee.org/iel5/6158724/6165526/06165539.pdf

So, if you've read to this point, thanks! Any and all insights/experiences as to the general application and reliability of epoxy resin type instrument transformers, especially at 72kV are appreciated. Any further insights as to our particular failure mode are also appreciated. I can imagine that a turn to turn fault that was too small to come out through winding resistance and turns ratio tests could eventually progress through heating to a winding failure. I can also imagine scenarios where a turn-turn fault leading to an arcing ground fault, and possibly a transient overvoltage on the secondary (thinking of the ungrounded neutral here) could also lead to a failure of this kind. This latter imagining would be a good dose of bad luck consisting of a double contingency of turn to turn fault AND an unlanded neutral ground.

JPC

 

[scottf]

JPC-

* Full disclosure - I work for an instrument transformer manufacturer that makes dry-type VTs up to 69kV and I'm not sure if the unit being discussed here is our unit or not. I looked in our quality system and don't see a record of this event being reported, so perhaps it is not our unit.

First point is that dry-type ITs can fail and, on occasion, it can be catastrophic. The extent and frequency of failures, of course, varies by design and manufacturer.

My company has been making dry-type 69kV VTs for a very long time and the in-service record is very good, with very few reported catastrophic failures. If you wish to share your contact info, I would be happy to give you more details.

As for the failure, the ungrounded secondary is worth attention. If a secondary winding does not have ground reference, the potential of the entire winding can float up to fairly high voltages by means of capacitive coupling between the primary and secondary winding. The secondary windings are tested to withstand 3kV to ground for 1 minute per standard. An ungrounded winding could exceed that voltage and lead to internal arcing likely between the secondary winding and core...at least in our design, as the insulation between the secondary and the H2 layer of the primary is tested to withstand 19kV for 1 minute (5kV class). If this arcing damages the turn-to-turn insulation or creates an inter-winding fault to ground, that leads to very high currents in the secondary and primary winding, leading to a lot of internal heat and ultimately a failure. That type of failure would be consistent with a secondary voltage reading that decreases as further turn-to-turn shorts occur due to thermal runaway.

 

[FacEngrPE]

Consider Ferroresonance a possibility. per

https://electrical-engineering-portal.com/ferroresonance-occurrence-in-power-transformer

reconnecting so that the neutral of the potential transformers is grounded will damp out the resonance.
Scottf's explanation provides the details of how these events fail the PT's I had not seen this particular detail previously.

This search link provides more discussion on ferroresonance in voltage transformers

https://duckduckgo.com/?q=ferroresonance+in+voltage+transformers&t=lm&ia=web

 

[prc]

1) The VT is connected to the utility delta connected 69 kV. Ungrounded?
2) Oil-filled VTs were working fine; the only problem was dry VT.
3) Check the working flux density used in the core of these two types of VTs at rated voltage and frequency. If VT is connected to the unearthed system, B should be limited to about 0.8 T . Please see whether the B of dry VT is higher than oil-filled VT.

 

[j_p_c]

Thanks all of you for your replies to date.
@scottf

I'd love to discuss this with you offline, what's the best way to direct message without sharing our private emails? One thing I wonder is why, with a high impedance ohmmeter, the voltage to station ground of the VT secondary wasn't elevated, when the circuit was initially checked, and before the ground was reconnected. The value read was perhaps 10% above expected secondary nominal.​

@FacEngrPE

Thanks, yes it has been considered. In the context of the original plant design assuming effective grounding from the utility side, but perhaps, despite being solidly grounded at that end, it actually is of a high impedance... In that sense, perhaps. This is why I noted utility solidly grounded.​

@prc

1) No. The VT is Yg:Yg, however the secondary was unintentionally left ungrounded​

2) Yes.​

3) An interesting suggestion. Just a note that the exact same VT models as that which failed are now back in service and operating safely so far. This does not appear to be an issue by design, and we are aware of many sites with this exact same configuration and VTs​